In many illumination systems, targeted areas to be illuminated are much larger than an emitting area of the light sources. Many artificial light sources emit light in an approximately Lambertian distribution. In many cases the Lambertian distribution emits light at high angles from 65 to 90 degrees relative to nadir. In office and other environments, it is often desirable to reduce or minimize light emitted in 65 to 90 degree angles, because of discomfort viewers may experience in directly viewing the lights from those angles, and/or because of reflections of light from those angles from displays, work surfaces, and other objects.
In some countries, specifications or recommendations regarding luminaires limit the amount of light in the 65 to 90 degree range. In the United States, ANSI/IESNA RP-1-04 recommends maximum limits for the luminous intensity emitted at angles above 65, 75, and 85 degrees (at any azimuthal angle). In Europe, EN-12464 places similar limits on luminance at high angles. In addition to specific standards, specifications, or recommendations, in some cases lighting designers will prefer luminaires with limited high-angle luminous intensity. In some cases, limited high-angle luminous intensity is desirable along one azimuthal plane (e.g. East-West) while not being required in the orthogonal plane (e.g. North-South). In many other cases limited high-angle luminous intensity is desirable in all azimuthal planes.
In many cases it is desirable to increase the axial luminous intensity of a light source, so as to most effectively light the space below the luminaire. In many of these cases, it is desirable to do so without artifacts at high angles such as a wink that might be caused by a 90-degree prism film.
A downward-facing light source with Lambertian light distribution has luminous intensity that is proportional to the cosine of the angle from nadir (the downward-facing direction). By definition, the Full Width at Half Maximum (FWHM) of a Lambertian distribution is 120 degrees. In the lighting industry, the term “Lambertian” is also frequently used to refer to light distributions with similar quality but of different widths. That is, distributions that have a peak at nadir, and monotonically decrease at higher angles are often called Lambertian. In one example, a Gaussian distribution with FWHM of 80 degrees will often be called “Lambertian” in the lighting industry. FIG. 1 shows a measurement of a wide, approximately Lambertian light source. High-angle luminous intensity is high, with luminous intensity at 65 degrees approximately 37.5% of the peak luminous intensity.
Herein, the term “high-angle luminous intensity” will refer to luminous intensity at polar angles between 65 and about 90 degrees relative to nadir.
Herein, the term “axial luminous intensity” will refer to luminous intensity at the polar angle of about 0 degrees. For most downward-facing lighting fixtures, the axial direction is straight down.
High-efficiency LED lighting is being increasingly adopted. Typical LED light sources emit light into a Lambertian distribution with a Full Width Half Max (FWHM) of approximately 120 degrees. Although LEDs with many other light distributions are available, many cost-effective LEDs sold for general lighting are of the 120 degree Lambertian variety. Many luminaires (LED and traditional) have flat outer surfaces (such as some downlights, task lights, and troffers). In many cases, light emitted by these fixtures has high-angle luminous intensity that is undesirably high. This is often true for luminaires employing other types of light sources in addition to LEDs, such as incandescent lamps, fluorescent lamps, organic light-emitting diodes (OLEDs), etc. In many of these fixtures, a simple flat diffuser (such as a microstructured, holographic, or volumetric diffuser) is used to diffuse the LEDs, hiding their appearance from viewers and smoothing the surface appearance of the luminaire. In the absence of other features such as baffles, louvers, focusing reflectors, focusing refractors, and bezels, these diffusers often give Lambertian distributions of various widths (most typically about 80 to 120 degrees). In such cases, the high-angle luminous intensity may be undesirably high.
A 90-degree linear prism optic has one smooth surface and the other one is textured by an array of parallel linear prisms with 45-degree sidewalls, as shown, for example, in U.S. Pat. Nos. 2,474,317 and 3,288,990, in which one or two layers of prism optics are used to increase brightness directly under a luminaire, and reduce high-angle luminous intensity. A film with the similar properties is described by Cobb in U.S. Pat. No. 4,906,070. Films such as described by Cobb, usually employing prisms with peak angle of substantially 90 degrees, are used extensively for brightness enhancement of the back light unit inside a display system. In both lighting and displays, a brightness-enhancing prism is used with the light entering smooth surface of the optic, and thus the prisms facing away from the light source. Rays incident perpendicular to the surface of the film will encounter total internal reflections (TIR) from the prisms. Those light rays are generally reflected back into the backlight, which is generally configured with high reflectivity to recirculate those rays back toward the prism film (sometimes repeatedly), until they enter the prism film at larger incident angle and are allowed to pass to the viewer of display. Rays incident at larger angles are at least in part refracted through the prisms, and on average over all angles, the average exit angles are smaller than the average entrance angles, when measured relative to the normal to the prism optic. The angle bending and recirculation process creates a narrower FWHM light distribution (approximately 70-95 degrees) than the incident Lambertian distribution (approximately 120 degrees), and axial brightness enhancement. Said another way, a prism illuminated by Lambertian light in this orientation and with appropriate recirculation will increase axial luminous intensity, while reducing the FWHM. At some polar angles between about 65 and about 90 degrees, luminous intensity is decreased, but most 90-degree prisms also produce a distinct bright band (sometimes called a “wink”) at some polar angles above about 65 degrees at some azimuthal angles. This wink can produce high-angle luminous intensity that is unacceptably high. A measured 90-degree prism film illuminated by an approximately Lambertian source is shown in FIG. 2, in which the measured azimuthal plane was perpendicular to the major direction of the linear prisms, the “wink” being the peaks noticeable at approximately +/−70 degrees. This wink and the light paths within a prism optic that lead to the wink are described, for example, by Richard et al. in U.S. Pat. No. 7,777,832. Having no wink is defined herein by having a light distribution that substantially monotonically decreases as polar angles increase from the angle of peak luminous intensity. Richard et al. describe incorporating diffusion into a linear prism film to make the wink less noticeable in displays, using what is essentially a blurring process. This process may leave too much high-angle luminous intensity for use in lighting applications. Thus it may be desirable to simultaneously have substantially no wink and have low high-angle luminous intensity.
Cones are known in the art to be a surface structure that can reduce high-angle luminous intensity of a light source. Such use of cone shapes is mentioned in U.S. Pat. Nos. 2,474,317, 3,349,238, 3,159,352, 3,483,366, U.S. Patent Application Publication No. 2013/0057137, U.S. Patent Application Publication No. 2010/0128351 and German Patent Application No. DE102006009325A1. A cone-like hexagonal pyramid is mentioned in German Patent No. DE202010002744U1. In U.S. Pat. No. 7,631,980 and International Publication No. WO 2005/083317A1, a cone with inverted tip is pictured that resembles a prism bent into a single ring.